Parainfluenza, Respiratory Syncytial, and Adenoviruses

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PARAINFLUENZA VIRUS

Parainfluenza viruses are important viral pathogens causing upper and lower respiratory infections in adults and children

Classification

Family Paramyxoviridae

Genus Members

Paramyxovirus Parainfluenza [PIV types 1,2,3,4]

Mumps virus

Newcastle Disease Virus [birds]

Sendai virus [mice]

Morbillivirus Measles virus

Canine Distemper Virus

Pneumovirus Respiratory Syncytial Virus (RSV)

Structure

Parainfluenza viruses are relatively large viruses of about 150-300 nm in diameter. They have a spherical or pleomorphic shape.

The RNA is negative sense, unsegmented and single stranded (ss)

The nucleocapsid core is filamentous or herringbone-like, has helical RNA tightly associated with Nucleoprotein (NP), and also Phosphoprotein (P) and Large protein (L)

These are enveloped viruses with a host-derived lipid bilayer, associated with two virus-specific glycoproteins:

Hemagglutinin-Neuraminidase (HN). This is a viral attachment protein, that also causes hemadsorption and hemagglutination

Fusion protein (F). The F protein forms spikes out from the envelope. It promotes the fusion of host and viral cell membranes which is an initial step in infection. It is synthesized as a biologically inactive form (F0), which is activated by proteolytic cleavage to an active form that has 2 subunits, F1 and F2, linked by a disulfide bond.

Matrix (M) protein, located just within the envelope, is hydrophobic

Isolation

Cell lines such as primary Rhesus monkey kidney epithelial Cells (PRMK), LLC-MK-2, and human embryonic kidney cells are used.

Cytopathic effects occur such as rounding, bridging, cell lysis, and syncytium formation.

Hemadsorption (due to the interaction of viral hemagglutinin with specific erythrocyte receptors on guinea pig red cells) can be observed at 4° C. This may be seen even before the appearance of cytopathic effects and has been used for early diagnosis (especially PIV-1 and PIV-3).

Pathogenesis

The first step in the infection cycle involves attachment of the virus to host cell sialic acid receptors. This is mediated by viral attachment protein, a function served by the HN glycoprotein.

Next, the F protein catalyzes fusion of the viral envelope and host cell membrane, resulting in uncoating and release of the nucleocapsid structure into the host cell cytoplasm.

For transcription and protein synthesis to occur, first mRNA is formed with the help of RNA-dependent RNA polymerase which must be supplied by the virus. The polymerase function is carried out by the P and L proteins, and possibly also the NP. The genome is replicated by formation of a full-length positive sense RNA template onto which a negative sense RNA is then transcribed.

Assembly of the nucleocapsid occurs and M proteins are then associated with the viral glycoprotein modified cell membranes. Mature virions are released from host cell membranes by budding.

Epidemiology and Transmission

The virus is ubiquitous; infections occur as epidemics as well as sporadically.

Parainfluenza viruses are sensitive to detergents and heat but can remain viable on surfaces for up to 10 hours.

Transmission occurs via the following routes:

Large droplets - person to person through close contact

Aerosols of respiratory secretions

Fomites (virus survives on surfaces)

Clinical Features

Incubation period is 2 to 6 days.

Most infections are asymptomatic, especially in older children and adults.

Primary infections and re-infections occur.

Most persons have had primary infections before the age of 5 yrs.

Reinfections are clinically less severe, most commonly involve the upper respiratory tract and occur throughout life.

Fever and a spectrum of respiratory infections are caused by PIVs:

Rhinorrhea/rhinitis, pharyngitis, cough, croup (laryngotracheobronchitis), bronchiolitis, and pneumonia.

Croup - the subglottic region becomes narrower and results in difficulty with breathing, a seal bark-like cough and hoarseness.

PIV types 1 and 2 most often cause outbreaks of croup in autumn/early winter, with an alternate year pattern. PIV-1 tends to attack children ages 2-6 years.

PIV-3 can cause croup, though less commonly than PIV-1 and 2 and is sporadic, without a particular seasonal occurrence [spring and summer also].

Primary infection with PIV 3 in young infants and children of less than two years of age is a common cause of bronchiolitis (RSV more common).

PIV-4 is associated with mild upper respiratory infections

Otitis media, parotitis, aseptic meningitis occur although they are rare.

Particularly severe and persistent infections are known to occur in immunocompromised children and adults; prolonged viral shedding is seen.

Clinical Diagnosis

Antigen detection

Radio-immunoasay, enzyme immunoassay, fluoro-immunoassay, and immunofluoresence methods are used for antigen detection.

Nasopharyngeal secretions are collected, from swabs or washings and transported in viral transport medium and on ice.

Shell vial assay is useful in detecting growth in 4-7 days. Hemadsorption can be noted before cytopathic effects. Immunofluoresence is confirmatory.

Antibody Detection

Serology uses hemagglutinin inhibition to demonstrate a difference between acute and convalescent levels. A 4-fold increase in antibody titers is considered positive. However, serologic diagnosis is of limited value because of the presence of nonspecific inhibitors and the antibody being heterotypic (antibody that is common to different PIV types as well as the mumps virus)

Treatment

There is no specific treatment. Supportive treatment for croup includes humidification of air and racemic epinephrine. Corticosteroids may be used in moderate to severe cases.

Immunity

Immunity following infection is short lived. The role of antibody is not clear since reinfection has been seen even with high levels of antibody.

Cell-mediated Immunity (CMI) is probably more important for limiting infection.

Infection control

Asymptomatic shedding is common, making it difficult to contain spread of infection. Hand washing and preventing contamination of surfaces with respiratory secretions are important for limiting nosocomial spread.

RESPIRATORY SYNCYTIAL VIRUS

Classification and structure

Family Paramyxoviridae, genus Pneumovirus. Infection of cells results in syncytium formation.

These are spherical or pleomorphic enveloped viruses (100-350 nm) with single-stranded, negative sense linear RNA.

The envelope has 2 glycoproteins:

F - fusion protein, is important for fusion of viral particles to target cells and fusing infected cells to neighboring cells to form syncytia.

G - which is highly glycosylated, is important for viral attachment to host cells

Antigenic variations in the type of G protein determine the subgroup (A or B).

RSV lacks H/N proteins unlike other members of the family Paramyxoviridae

Properties

These viruses survive on surfaces for up to 6 hours, on gloves for less than 2 hours. They rapidly lose viability with freeze-thaw cycles, in acidic conditions and with disinfectants.

Pathology and Pathogenesis

Virus attaches (via G protein) to cells of respiratory tract.

Infected cells undergo necrosis, also syncytia form through fusion.

Cell to cell transfer of virus leads to spread from upper to lower respiratory tract.

Smaller airways (bronchioles) become plugged with debris and mucin; bronchoconstriction also occurs. The host immune response also induces some of the pathological changes.

Epidemiology

RSV has a worldwide distribution and most children have had an RSV infection by age 4 years

Out breaks are seasonal occurring from late fall through spring (November to May)

The virus is transmitted via large droplets, through fomites and via hands

The virus enters through the eyes and nose

Viral shedding continues for less than 1 to 3 weeks but longer in immuno-compromised hosts

RSV is the most frequent cause of bronchiolitis but is an infrequent cause of croup

Clinical Features

Incubation Period: 4 - 6 days (range: 2 - 8 days)

Upper respiratory infection (‘bad cold’) in older children and adults:

Clinical features: fever, rhinitis, pharyngitis

Lower respiratory infection- Bronchiolitis and/or pneumonia may occur after the upper respiratory infection:

Clinical features: cough, tachypnea, respiratory distress, hypoxemia, cyanosis.

Cough can persist for 3 weeks.

In young infants one observes apnea, lethargy, irritability, poor feeding.

Radiological features: atelectasis, streaking, hyperinflation.

Severe infections occur in pre-term infants (especially less than 35 weeks gestation and those with chronic lung disease), children with cyanotic congenital heart disease, and immuno-compromised hosts.

Diagnosis

Nasal washings, nasal aspirates or swabs should be transported on ice.

Rapid Diagnosis: DFA, IFA, ELISA

Viral culture is carried out in cell lines such as HeLa, Hep-2, Monkey Kidney cells. Cytopathic effects are usually seen in 2-5 days. Shell vial technique is useful

Serology: neutralizing antibodies (by CF, immunofluorescence) is not very useful for young infants.

Shell Vial

This method of viral detection involves the growth of a monolayer of susceptible cultured cells on a coverslip at the bottom of a vial. Such layers of cells in a shell vial are obtainable commercially. An patient sample inoculum containing a virus such as respiratory syncytial virus or cytomegalovirus is spun down onto the surface of the monolayer. This improves the sensitivity and the speed of detection of the virus. After incubation of the cells, they are stained with a specific anti-viral antibody

Treatment

Treatment is usually supportive by the provision of fluids, oxygen, humidification of air, respiratory support, bronchodilators

Ribavirin , a guanosine analogue (aerosol) has been used with some efficacy but is reserved for only persons at high risk for severe disease.

Immunity

Humoral immunity

Neutralizing antibody to F and G proteins, IgA is also produced.

Level of neutralizing antibody does not correspond to neutralizing activity

Immunity is short lived therefore reinfections are common.

Newborns may have some innate immunity

IgE response occurs in some individuals and may be a marker for future airway hyper-reactivity.

Cell mediated

T cells. Cytokine production also contributes to illness.

Prevention of spread

Handwashing

Isolation and cohort nursing

Protective gear: gowns, gloves, masks and goggles

Active immunization. The inactivated vaccine is no longer used because it was associated with an increase in severity of disease. Other vaccine candidates are in trial phases.

Passive immunoprophylaxis. There have been encouraging results from trials using pooled hyperimmune globulin (RespiGam) as monthly injections to susceptible infants during the RSV season. Now, a monoclonal antibody against F protein has been synthesized (Palivizumab- marketed as Synagis). It is used to prevent disease in children who are at risk for severe RSV infection.

ADENOVIRUS

These viruses were named "adenovirus" because they were first isolated in 1953 from tissue cultures of human adenoidal tissue.

Classification

They belong to family Adenoviridae, genus Mastadenovirus.

Adenoviruses are further classified into 6 subgroups (A through F), based on hemagglutinating properties and DNA homology.

About 47 serotypes have been isolated from humans.

Types 40, 41 belong to subgroup F and are enteric pathogens.

Common serotypes are 1 - 8, 11, 21, 35, 37, and 40.

Structure

These are non-enveloped viruses with a diameter of 70-90nm.

The genome is made of linear double-stranded (ds) DNA with 2 major proteins.

The capsid is icosahedral, comprised of 252 capsomeres. 240 are hexons; at the vertices are 12 pentons, from which a fiber with a terminal knob projects. This complex is toxic to cells - causing rounding and death of cells through inhibition of protein synthesis. The fiber proteins determine target cell specificity.

10 structural proteins are known.

Pathogenesis and Replication

Virus primarily attacks mucoepithelial cells of the conjunctiva, respiratory tract, gastrointestinal and genitourinary tracts. Attachment to host cell receptor occurs via the fiber protein. The virus replicates in the cytoplasm of host cells, but viral DNA replicates within the host cell nucleus. Early and late phases of replication occur, followed by assembly and release of virions.

Three types of infections occur in target cells:

Lytic - cell death occurs as a result of virus infection (mucoepithelial cells)

Latent / persistent / occult - virus remains in the host cell, which is not killed (lymphoid tissues such as tonsils, adenoids, Peyers patches)

Oncogenic transformation - cell growth and replication continue without cell death. This is seen in hamsters, most often with group A viruses (see oncogenic virus section).

Adenovirus also replicates in associated lymphoid tissues, and subsequent viremia can cause secondary infection in visceral organs.

Inefficient (error-prone) replication of the virus results in many excess antigenic components. These are liberated into the culture fluid in vitro as soluble antigens and lead to formation of basophilic staining intra-nuclear inclusion bodies in cells.

Properties

Adenoviruses are stable in the environment and to low pH, bile, and proteolytic enzymes - These properties make it possible for them to replicate to high titers in the GI tract.

Clinical Syndromes

Almost half of adenoviral infections are subclinical

Most infections are self-limited and induce type-specific immunity

Incubation period is 2-14 days; for gastroenteritis usually 3-10 days

Different clinical syndromes have been described:

Eye

Epidemic Keratoconjunctivitis (EKC), acute follicular conjunctivitis, pharyngoconjunctival fever

Respiratory system

Common cold (rhinitis), pharyngitis (with or without fever), tonsillitis, bronchitis, pharyngoconjunctival fever, acute respiratory disease (LRI), pertussis-like syndrome, pneumonia- sometimes with sequelae

Genitourinary

Acute hemorrhagic cystitis, orchitis, nephritis, oculogenital syndrome

Gastrointestinal

Gastroenteritis, mesenteric adenitis, intussusception, hepatitis, appendicitis. Diarrhea tends to last longer than with other viral gastroenteritides

Rare results of adenovirus infections include- Meningitis, encephalitis, arthritis, skin rash, myocarditis, pericarditis, hepatitis. Fatal disease may occur in immunocompromised patients, as a result of a new infection or reactivation of latent virus

Epidemiology

Endemic, epidemic and sporadic infections occur. Outbreaks have been noted in military recruits, swimming pool users, residential institutions, hospitals, day care centers etc.

Transmission: Droplets, fecal-oral route (direct and through poorly chlorinated water), fomites

Many infections are subclinical

Infections are most communicable in the first few days of illness, however infective period continues since clinical infection may be followed by intermittent and prolonged rectal shedding

Secondary attack rate within families: up to 50%;

Adenovrius outbreaks are seasonal: Respiratory disease mainly occurs in late winter through early summer. Pharyngoconjunctival and EKC infections occur in the summer months while GI disease does not seem to be seasonal

Diagnosis

Clinical specimens, such as swabs (nasopharyngeal, conjuncticval, rectal, or other) and washings, corneal scrapings, stool, urine or biopsy and autopsy materials etc. should be transported in viral transport medium.

Viral Isolation in cell cultures is carried out in HeLa, human embryonic kidney (HEK) and human fetal diploid cells (HDFL). A549 cells lines are used for types 1-39.

Subgroup F (serotypes 40, 41) do not grow well in these cell lines, but do grow in Graham-293 (a modified HEK cell line).

Shell vial culture technique aids in faster detection.

Cytopathic effects include swelling and rounding of cells. Cells may become refractile and clustered into irregular clumps.

Isolation of virus from a pharyngeal specimen is more suggestive of a current clinical infection than from fecal specimen.

Rapid detection of enteric types (serotypes 40, 41) is by ELISA or immunofluorescnece antibody. Immune EM (aggregation with sera) may also be used

Other detection methods in current use include electron microscopy, polymerase chain reaction and nucleic acid probes.

Serology is mainly used for epidemiologic studies

Prevention

Handwashing

Contact precautions, respiratory precautions in health care settings

Adequate chlorination of swimming pools

Sterilization / disinfection of ophthalmologic equipment and use of single dose vials of ophthalmic medications

Vaccine: live, enteric coated, oral vaccine (types 4, 7, 21)